This invention relates generally to semiconductor devices and, more particularly, to stable ohmic contacts for a gallium arsenide semiconductor and a method for producing such contacts.
In the art of manufacturing semiconductor devices, contacts must be connected to or incorporated within the body of a semiconductor such that electrical connections can be made thereto. Preferably, a contact is made such that no rectifying barrier or junction is formed between the contact and the body of the semiconductor. Such a non-rectifying contact is referred to as an ohmic contact.
Ohmic contacts may be formed on the semiconductor by thermally alloying appropriate metals to and with the semiconductor at a selected contact area on the surface of the semiconductor. The alloy process is very sensitive to time duration and temperature, and the temperatures required are often so high that undesired contaminants are introduced into the contact, the semiconductor or both. This is a particular problem in field effect transistors (FET's) wherein an electrical contact must be made on an insulating gate of the semiconductor.
A method of forming ohmic contacts on semiconductors utilizing ion implantation is disclosed in U.S. Pat. No. 3,600,797. Ion implantation is a process which has been used for some time to implant impurity atoms in a semiconductor. The impurity atoms which are normally neutrally charged are ionized by inducing a predetermined electrical charge thereon. The ions are formed into an ion beam by electrical fields such that the size, shape and velocity of the ion beam can be controlled.
The ions are then made to enter the lattice framework of a semiconductor by directing the beam at the semiconductor, with the result that atoms of the impurity are positioned within the lattice of the semiconductor at desired positions. By controlling the direction and velocity of the ions in the beam, impurity atoms may be placed in a well defined region of the semiconductor.
Ion implantation may be direct where the ionized impurities are injected into the semiconductor or indirect where a layer of impurity material is initially applied to the semiconductor and then bombarded with ions of another material. For indirect implantation, the ions are electrically inert, i.e., a material which does not establish any particular type of conductivity in the semiconductor and which does not otherwise adversely effect the electrical or physical properties of the semiconductor. The momentum of the electrically inert ions serves to drive or deflect atoms of the impurity layer into the semiconductor. Direct and indirect ion implantation can be combined if electrically active ions are directed at an impurity layer of material on a semiconductor with the impurities from the layer and the ionized atoms both being injected into the semiconductor.
In the semiconductor contact forming method utilizing ion implantation as disclosed in U.S. Pat. No. 3,600,797, a layer of gold is initially deposited on a selected contact area of the semiconductor and the gold layer is irradiated with ions of an electrically inert material. Alternately, ions of an electrically active material, such as tellerium for n-type material, can be used to bombard the gold layer. In either event, gold atoms and atoms of the ionized bombarding beam are driven into the semiconductor to establish an ohmic contact for the semiconductor.
The literature teaches that gold based ohmic contacts to gallium arsenide tend to deteriorate upon aging due to the migration or spreading of the gold. See, for example, Electron Microscope Studies of an Alloyed Au/Ni/Au-Ge Ohmic Contact to GaAs by T.S. Kuan et al., J. Appl. Phys. 54 (12), December, 1983. Such deterioration of gold based ohmic contacts is amplified and accelerated when contacts are subjected to elevated temperatures or strong electrical fields. Accordingly, an ohmic contact which is initially formed by depositing gold on a GaAs semiconductor, even if atoms of the gold are implanted or mixed into the semiconducter, can be expected to result in a contact which has a rectifying barrier as the contact ages. It is thus apparent that the need exists for a stable ohmic contact for gallium arsenide semiconductors and a method for making such a contact.